Harvey murine sarcoma virus (Ha-MuSV) is a replication-defective transforming retrovirus composed of both mouse and rat genetic sequences. We have localized the sequences responsible for transformation to approximately 1.0 kilobases in the 5' half of the viral genome. We have also detected these sequences in the high-molecular weight DNA of both mice and rats. This project has involved recombinant DNA technology and other biochemical analyses.